In other words, a lot of media does not begin at the \textit{beginning} point and will not be properly rendered until enough data has been read to assemble a picture.
You can increase the thumbnail size, clarity of pixels (memory size) and color mode but it takes a lot more memory.
Change these values in \texttt{Settings $\rightarrow$ Preferences}, Appearance tab, right hand side of the Layout section -- be aware that when you click OK, your session will re-initialize.
-You can also temporarily increase the preview mini-window by use of the mouse wheel up or down.
+You can also temporarily increase the preview mini-window by use of the mouse wheel up or down for media but not for clips.
There are 4 options for the preview mode.